Dynamic range is an important characteristic of electronically steered antenna array systems based on a multi-channel digital receiver configured for detecting, processing and estimating parameters of two or more high and low power level signals received from several simultaneously operating emitters. Instantaneous dynamic range of a multi-channel digital receiver operating with signals originating from simultaneously operating plurality transmitters is typically defined as the maximum ratio of the highest and lowest power level signals that may be detected such that the signal parameters can be accurately estimated. Enhancement of the receiver instantaneous dynamic range is increasingly important in applications where accurate digital signal processing, originating from lower level transmitters, is required.
Referring to FIG. 1, a general schematic block diagram of a prior art multi-channel digital receiver 10 is illustrated. The receiver 10 includes an antenna array 11 of antenna elements 12 connected to a plurality of parallel receiver channels 13 arranged downstream of the antenna array 11, a digital beam forming unit 14 arranged downstream of the receiver channels 13, and a signal processor unit 15 arranged downstream of the digital beam forming unit 14. Each receiver channel 13 includes an analog amplifier 131 and an analog-to-digital converter (ADC) 132.
In operation, the ADC 132 samples the channel signal and produces an ADC channel output that can be characterized by a sampling frequency and by a number of bits. The digital beamforming unit 14 receives the ADC channel outputs from the multiple channels and computes a digital beam by multiplying each channel by beamforming weights and computing sums. As a result of the beamforming process, signals arriving from a certain specific direction are selected and amplified, while all signals arriving from other directions are attenuated. In addition to the beamforming process, unwanted signal from another certain direction can be suppressed and removed by steering beam pattern nulls in this direction, while maintaining the main lobe in the selected direction.
Usually, a relatively large number, e.g., up to a thousand or possibly more digital receiver channels may be required to form narrow beams for high spatial selectivity of such a receiver.
Generally, there is a relation between the number of bits and the instantaneous dynamic range of analog-to-digital converters (ADCs). According to this relation, a higher instantaneous dynamic range is obtained for the ADCs with higher quantization levels (i.e., with a higher number of bits) in the output. However, wideband ADCs with a large number of effective bits usually require high power consumption in signal processing. Moreover, such ADCs with a high number of quantization levels are usually complex and costly to implement. Accordingly, implementation of high dynamic range multi-bit wideband ADCs in the multi-channel system, as shown in FIG. 1, is not practical.
On the other hand, the use of ADCs with a high sampling rate and a low number of bits in a multi-channel receiver may result in low power consumption, and provide a simple and cost effective solution. However, the dynamic range of ADCs with a low number of bits usually suffers from high level quantization errors that add quantization noise to the output. As a result of quantization errors, together with high order harmonics of the strong signal, a noise is produced which can cause misdetection of low level signals received simultaneously with high power level signals. Additionally, these high order harmonics can create false alarm detections.
One of the possible solutions for improving the dynamic range of the analog to digital conversions is to use dithering, also known as quantization error dispersion. In general, the technique involves adding a dither signal to an analog signal prior to its conversion to a digital value in order to decorrelate the quantization noise from the input signal and randomize quantization error, thereby to enhance the resolution of the analog to digital converter. The dither signal itself can, for example, be random white noise, band limited random noise, a periodic ramp, square wave, triangle wave sweep, etc.
FIG. 2 shows a schematic block diagram illustrating a conventional AD conversion circuit 20 where a dither signal is added to the analog input signal. The AD conversion circuit 20 includes a signal combiner 21, an AD converter (ADC) 22, a dither signal generator (DSG) 23 and a dither signal remover 24. In operation, the analog dither signal is added to the analog input signal before conversion to digital form. The lesser the ADC's bit number, the greater the dither level must be. The combined signal is then converted via ADC 22. Care must be taken when choosing the level of dithering to ensure that that the dynamic range is not compromised owing to the increase in the level of noise in the ADC. After conversion, further digital signal processing, for example, a fast Fourier transform (FFT), may be required to minimize desensitization (i.e. loss of sensitivity) caused by the dither signal. Thus, suppression of the dither signal is usually implemented in the signal post processing after the analog to digital conversion in order to minimize impact on the sensitivity of the output. In particular, suppression of the dithering signal can be based on the knowledge of specific characteristics of the dithering signal. Accordingly, the dither signal can finally be removed digitally by the dither signal remover 24 to produce a digital output signal.
Referring to FIG. 3, a general schematic block diagram of a multi-channel digital receiver 30 is illustrated, according to another example. In order to improve the performance of the beamforming multi-channel digital receiver shown in FIG. 1, each receiver channel 13 further includes a channel dither signal generator (DSG) 31 together with a channel signal combiner 32 arranged upstream of an analog-to-digital converter (ADC) 33. In operation, adding individual dither signals in each channel improves dynamic range performance of this channel of the multi-channel digital receiver.